*3.2. Experimental Procedures of Carbon Flux Measurement*

The low-pressure vacuum carburizing furnaces were heated to 920 ◦C, 950 ◦C, and 980 ◦C. The pretreated samples were transferred into a hot chamber from a cold chamber and kept warm for 20 min to ensure they were fully heated. Then, the carburizing process was performed. The pressure in the carburizing furnaces was rapidly increased to 100, 200, and 300 Pa with a large flow setting. The closed-loop control of the carburizing pressure was realized by controlling the pumping speed and air intake volume of the root pump to maintain the pressure within the range of ±10 Pa. After stabilizing the pressure for 30, 60, 90, 120, and 150 s, the carburizing gas was quickly extracted, the samples were taken out of the furnaces, and then immediately quenched and weighed after cleaning. The schematic of the pressure maintaining process in the boost stage is shown in Figure 6.

**Figure 6.** Schematic of the boost stages of the carburizing process.
